"Peaking units would typically be your small, combustion turbine units, the older, less-efficient units," Wallace says. "They would also be some of the hydro units that are very small that aren't running all the time."

Wallace says, the heat means, more heavily-used "base power" units like coal and nuclear plants are now running nearly all of the time.

She says, one way for customers to reduce their energy bill is to turn thermostats to 78 degrees, use thermostat timers and use ceiling fans to move air around.